Compressed under ambient temperature, graphite undergoes a transition at approximately 17 gigapascals. The near K-edge spectroscopy of carbon using synchrotron x-ray inelastic scattering reveals that half of the pi-bonds between graphite layers convert to sigma-bonds, whereas the other half remain as pi-bonds in the high-pressure form. The x-ray diffraction pattern of the high-pressure form is consistent with a distorted graphite structure in which bridging carbon atoms between graphite layers pair and form sigma-bonds, whereas the nonbridging carbon atoms remain unpaired with pi-bonds. The high-pressure form is superhard, capable of indenting cubic-diamond single crystals.

A number of recent transport and magnetization studies have shown signs of ferromagnetism in the LaAlO 3 /SrTiO 3 heterostructure 1-6 , an unexpected property with no bulk analog in the constituent materials. However, no experiment thus far has provided direct information on the host of the magnetism 7-11 . Here we report spectroscopic investigations of the magnetism using element-specific techniques, including x-ray magnetic circular dichroism and x-ray absorption spectroscopy, along with corresponding model calculations. We find direct evidence for in-plane ferromagnetic order at the interface, with Ti 3+ character in the d xy orbital of the anisotropic t 2g band. These findings establish a striking example of emergent phenomena at oxide interfaces.Recent advances in the atomic-scale synthesis and characterization of perovskite oxide heterostructures have engendered significant interest in their electronic and magnetic structure. SLAC-PUB-15439Division of Materials Sciences and Engineering, under contract DE-AC02-76SF00515 and BES.Given their vast physical properties in bulk form, and their epitaxial compatibility, perovskites provide an ideal arena to explore the competition, interaction, and creation of many ground states at their interfaces 12 . The LaAlO 3 /SrTiO 3 heterostructure is a canonical example, exhibiting interface conductivity 13 , superconductivity 14 , and ferromagnetism 1-6 at the interface between two wide band-gap insulators. From a fundamental perspective, ferromagnetism is perhaps the most important property; although bulk SrTiO 3 can be doped to be metallic and superconducting, neither constituent in bulk form exhibits ferromagnetism. Hence interface ferromagnetism here could be a leading example of truly emergent phenomena. Most previous studies used bulk probes (macroscopic magnetization or torque) 3, 4 ; while scanning SQUID microscopy could localize the magnetism to the near surface region 5, 15 , the specific location where the moments reside is beyond the resolution of the probe. In principle, magnetism could arise from cation/anion defects in the LaAlO 3 or SrTiO 3 , or could be specific to the interface; theoretical scenarios have been proposed for all of these mechanisms 7-11 . Thus it is of central importance to determine the microscopic nature of the observed ferromagnetism.To address this issue, we applied element-specific techniques at the LaAlO 3 /SrTiO 3 (001) interface, namely synchrotron radiation based x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) (see Materials and Methods section). These measurements can uniquely determine whether the observed magnetization is due to a magnetic moment ( ) from one of the constituent elements, or from extrinsic impurities. All spectra were acquired by recording the total electron yield (TEY). Since the maximum probing depth of TEY is approximately 5~10 nm, these measurements are very sensitive to the interface with proper choice of LaAlO 3 thickness. Using the angle dependence of the XMCD signal, whic...